PHP: Avoid undefined index?

Question

Every time a POST value is not equal to the list of values set in an array will return: Undefined Index error, I made an if statement but is not working.

Here's the if statement:

 if ($products[$_POST['product']] == $_POST['product']) {
do everything;}
else {
echo "This item is not available";
}

EDIT2:

Seen the current situation avoiding the warning wont help much because I'm dealing with several factors, for example a list of items in a Shopping Cart, if the invalid product is not removed, it will be added to the shopping list session.

This is the full script:

<?php

session_start();

//Getting the list
 $_SESSION['list'] = isset($_SESSION['list']) ? $_SESSION['list'] : array();    

 //stock    
 $products = array(      
     'Pineaple' => 500, 'Banana' => 50, 'Mango' => 150,       
     'Milk' => 500, 'Coffe' => 1200, 'Butter' => 300,      
     'Bread' => 450, 'Juice' => 780, 'Peanuts' => 800,      
     'Yogurt' => 450, 'Beer' => 550, 'Wine' => 2500,    
 );    

if( isset($_POST['product']) ){


     //Saving the stuff    
     $new_item = array(      
         'item' => $_POST['product'],       
         'quantity' => $_POST['quantity'],     
         'code' => $_POST['code'],      
         'price' => $products[$_POST['product']] * $_POST['quantity'],    

     );



    $new_product = true;    
    foreach($_SESSION['list'] as $key => $item) {      
        if ($item['item'] == $new_item['item']) {        
        $_SESSION['list'][$key]['quantity'] += $new_item['quantity'];        
        $_SESSION['list'][$key]['price'] = $products[$new_item['item']] * $new_item['quantity'];        
        $new_product = false;
        }    
    }   

    if ($new_product) {      
        $_SESSION['list'][] = $new_item;        
    }    

    /*if ($new_item['item'] != $products[$new_item['item']]) {
        echo "This item is not available";}*/

    //listing    
    echo  "<b>SHOPPING LIST</b></br>";    
    foreach($_SESSION['list'] as $key => $item) {       
        echo 'Product .'. $key. ' '. $item['item'], ' ', $item['quantity'], ' units: ', $item['price']. '<br />';    
        }

}

else {
echo "This item is not available";
}

echo "</br> <a href='index.html'>Return to index</a> </br>";

//Printing session
var_dump($_SESSION);

session_destroy();

?>

Answer 1

With PHP 7, the null coalescing operator can be used to deal with optional request variables. You can change your $_POST['product'] reference to $_POST['product'] ?? null which will resolve to null (rather than throwing the warning) if 'product' is not a valid key in the post array. If you wanted to check both the $_POST and $_GET arrays for a value, you would use $_POST['product'] ?? $_GET['product'] ?? null.

Answer 2

I'm a bit confused by your code. It looks like your array has the same key and value, so:

$products['saucepan'] = 'saucepan'

Perhaps you are trying to do this, which will check whether the product exists in the products array:

if(isset($_POST['product']) && array_key_exists($_POST['product'], $products))
{
  // do stuff
}
else
{
  echo "This item is not available";
}